1. Technical Field
The present invention relates to a vibration gyro element, a vibration gyro sensor, an electronic device, and a method of detecting physical quantity of a vibration gyro sensor.
2. Related Art
When angular velocity is applied to an object moving at a predetermined velocity, a Coriolis force is generated in the direction perpendicular to the plane determined by the axis of the direction (the velocity direction) of movement and the axis of the direction (the angular velocity direction) of the angular velocity vector. In the vibrating gyro sensor, the angular velocity is obtained based on the electrical signal varying in accordance with the Coriolis force. Specifically, the vibrating arm is exited to generate a driving vibration. When the angular velocity is applied, the detecting vibration is caused in the vibrating arm in the direction perpendicular to the direction of the driving vibration by the Coriolis force corresponding to the angular velocity. In the vibration gyro sensor using a piezoelectric material, the electric field is generated due to the distortion (the stress) caused in the vibrating arm by the detecting vibration, thereby migrating the charge. The variation in the electrical signal (i.e., a minute current signal) caused by the migration of the charge is proportional to the level of the angular velocity applied thereto. Therefore, by detecting the variation (e.g., the variation in the amplitude of the direct-current voltage) in the electrical signal, the angular velocity applied thereto can be detected.
As a vibration gyro element (a vibrator element used for the vibration gyro sensor) for constituting the vibration gyro sensor, there is known a so-called double T vibration gyro element (see, e.g., JP-A-2004-245605 (Document 1)). In the double T vibration gyro element, roughly T-shaped driving vibration systems are arranged symmetrically about the detection vibration system located at the center. In the double T vibration gyro element, the Coriolis force caused in the drive arm by the angular velocity acting around the Z-axis is propagated to the detection arm via the support arm and the base section.
Further, in JP-A-2006-250769 (Document 2), there is proposed a structure of reducing the energy loss in the double T vibration gyro element. The vibration gyro element disclosed in Document 2 is composed of a piezoelectric material having a trigonal crystal structure. The vibration gyro element has a detection arm in the Y-axis direction, and a pair of drive arms extending in directions at angles of +120° and −120° respectively with the Y-axis. The drive arms vibrate in the X-Y plane, and if the angular velocity acts thereon around the Z-axis, the Coriolis force is generated in the extending directions of the drive arms. The detection vibration in the X-axis direction is caused in the detection arm by the X-axis direction component of the Coriolis force.
The vibration gyro element described in Document 1 or Document 2 is for detecting the angular velocity acting around the Z-axis, and cannot directly detect the angular velocity acting on the X-axis or the Y-axis. Therefore, in order for detecting the angular velocity acting on the X-axis or the Y-axis using this gyro element, it is required to arrange the element so as to erect vertically. Therefore, the size in the thickness direction of the sensor is large (difficult to be low-profile), and the mounting cost also increases.
The gyro element for detecting the angular velocity acting on the X-axis or the Y-axis is described in, for example, JP-A-5-256723 (Japanese Patent No. 3,007,216) (Document 3).
In the vibration gyro element described in Document 3, a common (integrated) vibrating arm extends in a predetermined direction from the base section. The tip portion of the vibrating arm is forked into two. At the tip portion of each of first and second arms thus formed in a two-forked manner, there is formed a drive electrode. Further, on the side of the vibrating arm near to the base section, there is formed a detection electrode. The tip portion of the common vibrating arm forked into two is exited with the voltage applied to the drive electrodes, thereby causing the driving vibration in the crystal plane (the X-Y plane) of quartz crystal or quartz constituting the vibrating arm. When the angular velocity is applied around the axis (around the Y-axis) of the extending direction of the drive arm, the Coriolis force acts perpendicularly to the crystal plane, and the detection vibration is caused in a direction (the Z-axis direction) perpendicular to the crystal plane.
Further, in recent years, attention is focused on the technology of using the micro electromechanical system (MEMS) technology to realize a small-sized and highly accurate vibration gyro element (the vibrator element for the vibration gyro sensor).
In the vibration gyro element described in Document 1, the Coriolis force (i.e., the detection vibration) caused in the drive arm is transmitted to the detection arm via the support arm and the base section. Since the detection vibration is attenuated while being propagated through the support arm and the base section, it has to admit that energy loss is caused in the propagation process of the detection vibration.
Further, in the vibration gyro element described in Document 2, the pair of drive arms extends in the directions at angles of +120° (at angle of −60° with the −Y-axis) and −120° (at angle of +60° with the −Y-axis) with the +Y-axis, respectively. Since the Coriolis force is generated in the direction perpendicular to the plane determined by the direction (the velocity direction) of the driving vibration and the axis of the angular velocity vector (the angular velocity direction), the force generated in the X-axis direction and for exiting the detection vibration becomes to have the smaller amplitude obtained by multiplying by sin 60°.
Further, the problem in the vibration gyro element described in Document 3 will be described below. FIGS. 19A through 19C are diagrams for explaining a vibration gyro element in the related art. FIG. 19A is a perspective view showing the structure of the vibration gyro element described in Document 3, and FIGS. 19B and 19C show examples of the cross-sectional view of the drive arm along the line A-A′ in FIG. 19A.
The vibration gyro element 900 shown in FIG. 19A has two drive arms 901, 903. In each of the drive arms 901, 903 of this vibration gyro element 900, ideally, the driving vibration is generated in the X-axis direction, the Coriolis force in the Z-axis direction is caused by the angular velocity around the Y-axis, and the detection vibration is caused in the Z-axis direction. However, in reality, due to the presence of such a manufacturing error as shown in FIGS. 19B and 19C, the vibration in the Z-axis direction is mixed in the driving vibration in the X-axis direction. In the example shown in FIG. 19B, due to the generation of a fin section in the drive arm, the direction of the actual driving vibration is shifted from the direction of the ideal driving vibration. Further, in the example shown in FIG. 19C, due to misalignment, unconformity is caused in the drive arm, thereby shifting the direction of the actual driving vibration from the direction of the ideal driving vibration.
The vibration in the Z-axis direction mixed therein is detected by the detection electrode as the detection vibration, and the angular velocity output is generated despite that no angular velocity is applied. Specifically, in FIGS. 19B and 19C, the mechanical leakage component represented by the thick dotted line is caused even if the actual angular velocity is zero. Therefore, despite that no angular velocity is actually applied, the electrical signal corresponding to the mechanical leakage component is output. This is the failure by the mechanical coupling, and the error component thereof is extremely large compared to the output by the Coriolis force, and the imbalance in the driving vibration affects the detection accuracy of the vibration gyro element. Further, the mechanical coupling is varied by the ambient temperature, and therefore, significantly affects the temperature stability of the gyro sensor.